Susan Phillips, an anthropologist at Pitzer College, wasn’t looking for markings left by men 100 years ago. She was looking for graffiti of a more recent vintage. But during her exploration of the bridges and walls of the Los Angeles River, she came upon graffiti drawn in the 1910s by itinerant men, the Associated Press reports.

“Hobo graffiti” was a code that let travelers tell each other where they were headed and what conditions were like at this location. The pieces that Phillips found included stylized arrows—“little heart things”—that pointed the direction a person was headed. She found messages from men with names like Oakland Red and Tucson kid, dated back to Aug. 13, 1914, and July 1, 1921.

The most notable graffiti is the signature “A-No. 1.” A-No. 1, or Leon Ray Livingston, was one of the most famous hobos of his time; he wrote twelve books on hobo life. Maybe he left his mark here. Or maybe someone else was just using his traveling name, which happened from time to time.

Hobo graffiti, etched into stone or drawn with grease pencil, has mostly disappeared by now. The graffiti here was preserved after the Los Angeles river was lowered 25 feet, making the bridge more inaccessible both to people and the elements. Phillips is looking for a way to capture it—probably digitally, she told the AP. Graffiti’s by nature ephemeral, and eventually these messages will disappear from these walls as well. Some already have.

Bonus finds: Mating snakes hanging from the ceiling

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